Metal organic molecular beam epitaxy (MOMBE) apparatus

ABSTRACT

The invention relates to apparatus for epitaxial processing using metal organic molecular beams. The MOMBE apparatus employs a manifold for supplying metal organic vapor to a reactor which is operated under vacuum. The manifold includes a bubbler in which MO vapor is formed and mixed with a carrier gas. The bubbler provides flexible, three parameter control of the MO reagent permitting use with MO reagents of low vapor pressure. A compensation flow is provided parallelling the reagent flow and employing four valves which are ganged and switched so as to supply the MO carrier gas mixture either to the reactor line or to the vent line and maintain equal flows and pressures during this switching operation. The apparatus is capable of forming very thin reproducible epitaxial layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the processing of epitaxial layers uponcrystalline substrates, and more particularly to an apparatus forgrowing epitaxial layers by means of a metal organic molecular beam.

2. Prior Art

The Metal Organic Molecular Beam Epitaxy ("MOMBE") technique is used togrow epitaxial layers upon a semiconductor substrate in the formation ofsemiconductor devices. The growth takes place upon a heatedsemiconductor substrate in a growth chamber into which one or morereagents in a gaseous or vapor state are introduced.

The MOMBE reagents are converted to a gaseous or vapor state in abubbler by passing a carrier gas through a metal organic reagent inliquid form. These reagents are suitable for epitaxial formation and arecapable of saturating the small flow of carrier gas as it passes throughthe bubbler. The carrier borne gaseous MO compounds are then injectedinto the growth chamber where they decompose into "metallic" and organiccomponents upon encountering the heated substrate. Ideally, the organiccomponent escapes and is removed by the vacuum pumps while the"metallic" component bonds to another "metallic" component on the heatedsubstrate to form the epitaxial layer.

The letter "M" in the acronyms "MOMBE" and "MO", accordingly stands for"metal" but includes elements from Groups II to VI. Group III metals,which bond with Group V semi-metals, form the III-V semiconductorcompounds which are more common. Among the III-V compounds are galliumarsenide, aluminum gallium arsenide, gallium indium arsenide, indiumphosphide, gallium phosphide, gallium indium phosphide, gallium indiumarsenide phosphide, and indium antimonide.

Epitaxial processing, when properly carried out, produces crystallinelayers which have uniform lattices, accurate impurity distributions, andaccurately gauged thicknesses.

A plurality of differing layers may be required in common semiconductordevices. For instance, in a high electron mobility transistor (HEMT),the final structure consists of five discrete layers, each optimized toenhance transistor performance. The layers include the substrate, whichis 20 mils thick, which may be of indium phosphide. The firstepitaxially formed layer is also of indium phosphide, and is one micronthick. The first epitaxial layer is followed by one 800 Å layer ofgallium indium arsenide, a 400 Å layer of aluminum indium arsenide witha 45 Å undoped underportion, and a final 200 Å layer of gallium indiumarsenide.

The performance of the HETT and devices optimized for high frequencyperformance depends on accuracy-ideally to within an atomic layer - (2-3Å) in the uniformity of the thickness of each of the several epitaxiallayers. In addition it is desired that the transitions between layers -the hetero-interfaces - be abrupt. The present apparatus is intended toprovide means for forming layers of this quality and multiple layerstructures with abrupt hetero-interfaces.

The MOMBE technique incorporates the key advantages of two priortechnologies; MOVPE (metal-organic vapor-phase-epitaxy) and MBE(molecular-beam-epitaxy).

MOVPE reactors generally provide good control and reproducibility of themolar flow of metal organic reagents. However, hydrodynamic processessuch as gas-phase depletion, convection and turbulence occur in MOVPEreactors where the growth chamber pressures are typically from 0.1 tounity atmospheric pressure. At these pressures, the flow of the injectedgases is viscous and can become turbulent, which limits the accuracy,uniformity and reproducibility of the epitaxial layers and of devicesfabricated from these layers.

In MBE, hydrodynamic problems are eliminated by use of a vacuum(molecular flow regime) environment in the growth chamber using solidsources. However the uniformity, reproducibility and throughput areunfavorably affected by depletion effects in conventional solid sources.

MOMBE combines the accurately metered and long lived MO gas sources ofMOVPE with the vacuum environment of MBE and has the potential for ahigher uniformity, reproducibility and thoughput than eitherpredecessor.

Apparatus optimized to carry out the MOMBE process is accordinglynecessary for efficient MOMBE processing. Central to such apparatus isthe manifold for delivery of MO reagents to the MOMBE growth chamber.

In particular, the manifold in MOMBE processing must maintain reasonablyhigh MO molar flows with modest total gas flows. In MOMBE, the pressurein the growth chamber must remain below 10E-4 torr in order to maintainthe molecular flow regime essential to uniformity i epitaxial layerformation. If excessive carrier gas flows are used, then the pumpingspeeds required to evacuate the reactor chamber to the required lowpressure become prohibitive.

The maximum permissible total gas flow into the MOMBE chamber isapproximately 50 standard cubic centimeters per minute (sccm), assumingcurrently available pumping speeds. In an MOVPE system, total gas flowsof 10 standard liters/min (slm) (200 times greater than in MOMBE) arecommon and carrier flows in each bubbler are typically 50 to 100 sccm.When MO reagents having a low vapor pressure are used in MOVPE, thebubbler flow may be 400 sccm or even higher if an acceptable epitaxialgrowth rate is to be achieved. It is not possible to use such highbubbler flows in MOMBE processing. Accordingly, if low vapor pressurematerials are to be used in MOMBE, the process conditions must bealtered from the MOVPE processing conditions.

In short, in MOMBE processing, a manifold is required, which will injectadequate molar flow rates of the MO reagent, while using low carrierflow rates. In addition, the manifold should permit accurate metering ofthe MO molar reagents, and should accommodate reagents having low vaporpressures.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an improvedmanifold for MOMBE processing.

It is another object of the invention to provide a manifold for MOMBEprocessing, having improved accuracy in metering epitaxial reagents.

It is still another object of the invention to provide a manifold forMOMBE processing, in which low vapor pressure reagents may be employedwithout requiring excessive pumping capacity for maintaining the MOMBEchamber at the vacuum required for molecular beam propagation.

It is a further object of the invention to provide a manifold for MOMBEprocessing in which transients occuring as the reagents are turned onand off are minimized to facilitate formation of uniform andreproduceable, thin epitaxial layers.

These and other objects of the invention are achieved in a novel MetalOrganic Molecular Beam Epitaxy (MOMBE) apparatus comprising a manifoldand a growth chamber.

The manifold, which is coupled to a supply line for carrier gas,includes a first and a second adjustable mass flow controller, eachcoupled to the supply line and a bubbler with a chamber for avaporizable liquid reagent having an inlet coupled to the secondcontroller (set to a mass flow rate (Fc)) and opening beneath thechamber filling level, an outlet disposed above the filling level toremove carrier gas borne reagent, the chamber beingtemperaturecontrolled to establish a desired vapor pressure (Pa) for the liquidreagent.

The manifold further includes a pressure sensor coupled to the bubbleroutlet, and an adjustable needle valve also coupled to the bubbleroutlet, the needle valve adjustment setting the bubbler outlet pressure(Pc) to a desired value.

The foregoing elements provide three parameter control of carrier bornereagent flow (Fa) in accordance with the following expression

Fa=Fc/(Pc/Pa-1).

The manifold further includes a vent line, a reactor line, and fourganged valves, the first pair of ganged valves opening together as thesecond pair of valves closes together. The valves are connected tosupply reagent to the reactor line while a compensation flow is beingsupplied to the vent line, and vice versa. Adjustment of the mass flowcontrollers for equal gas flows thus maintains the total flow of gasinto said vent and reactor lines substantially constant to minimizetransients in the supply of reagent to the MOMBE reactor.

The MOMBE growth chamber includes an injector to which the reactor lineis connected, and means to support and heat the substrate to facilitateformation of the epitaxial layer with carrier gas borne reagent. Thegrowth chamber is evacuated at a rate which maintains a vacuum of 10⁻⁴torr during admission of carrier gas to permit molecular beampropagation during epitaxial processing.

DESCRIPTION OF THE DRAWINGS

The inventive and distinctive features of the invention are set forth inthe claims of the present application. The invention itself, however,together with further objects and advantages thereof may best beunderstood by reference to the following description and accompanyingdrawings, in which:

FIG. 1 is an illustration of a portion of a manifold for use in a metalorganic molecular beam epitaxy (MOMBE) apparatus designed to control thesupply of a carrier gas borne metal organic reagent to the MOMBE growthchamber, the control being achieved by adjustment of three parameters;

FIG. 2 is an illustration of a larger portion of a MOMBE manifold usedto supply carrier gas borne metal organic reagent to a single injectorof a MOMBE apparatus, the larger portion including a separatecompensation flow to reduce transients arising from switching thecarrier gas borne metal organic reagent on and off in injection;

FIG. 3 is an illustration of a MOMBE manifold supplying too metalorganic reagents to a single injector of a MOMBE growth chamber; and

FIG. 4 is a simplified showing of a MOMBE growth chamber in which twoinjectors are illustrated, each of which may be supplied with reagentfrom a separate MOMBE manifold.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a portion of a manifold for usein a metal organic molecular beam epitaxy (MOMBE) apparatus and in whichthree parameter control of reagent injection is provided. Theillustrated portion includes a manifold input line 11 to which a carriergas, typically hydrogen at atmospheric or somewhat below atmosphericpressure, is supplied. The useful output of the manifold, which containsa carrier gas borne metal alkyl reagent, for example, is discharged intoa reactor line 12. The reactor line 12 is coupled to an injector (notshown in this figure) for supply of the carrier gas borne reagent to aMOMBE growth chamber. The manifold also discharges into a vent line 13,where the flow of metal organic reagent can be stabilized and maintaineduntil required for injection.

The portion of the manifold illustrated in FIG. 1 includes a mass flowcontroller 14, a metal organic reagent bubbler 15, which is charged withthe liquid metal organic reagent, a capacitance manometer 16, a needlevalve 17, and a pair of air operated valves 18 and 19 which are gangedso that when one member of the pair is opened, the other member isclosed.

The inlet of the mass flow controller 14 is coupled to the supply line11 to admit carrier gas. The supply line provides carrier gas atapproximately atmospheric pressure. The mass flow controller 14 may bemanually adjusted to set the flow of the carrier gas to a desired rate.Once set, the controller 14 automatically stabilizes the rate of flow towithin plus or minus one half of a percent against customary upstream ordownstream disturbances. The output pressure of the mass flow controller14 is substantially less than the supply line pressure, with a typicalrate of flow of 5 standard cubic centimeters per minute (sccm). Thesetting of the rate of flow is dependent on the reagent selected, andthe requirements of the epitaxial process. The outlet of the mass flowcontroller is coupled to an inlet of a bubbler 15 containing the liquidmetal organic reagent.

The bubbler 15 comprises a sealed cylindrical chamber 20, partiallyfilled with the liquid reagent, set within a temperature controlled bath32. The outlet of the mass flow controller 14, is coupled to the bubblerinlet 21, which opens into the chamber below the surface of the liquidand near the bottom of the chamber. This arrangement allows the carriergas to bubble up through the reagent to facilitate saturation of thecarrier gas by the reagent. An outlet 22 is provided having an openingin the chamber above the surface of the liquid. The outlet 22 couplesthe saturated carrier gas borne gaseous reagent to the capacitancemanometer 16. The manometer 16 is connected via additional tubing to themanual needle valve 17.

The liquid content of the bubbler chamber is a vaporizable metal organicliquid which dissociates at a suitably high temperature into a metallicand an organic component as earlier discussed. Group III metals, Group Vmetals, p-dopants and n-dopants may be introduced into the growthchamber using metal organic liquids as sources.

Ordinarily separate injectors are used for each of the four classes ofreagents. However, each injector may be required to process severalreagents from each class. In such a case, there may be two or more pathsin the manifold each traversing a separate bubbler to supply two or morereagents to one injector. (For clarity, a "manifold" supplies oneinjector, and may have more than one path through more than one bubblerleading to that injector. A growth chamber with plural injectors mayrequire a like plurality of manifolds.) A suitable metal organic liquidfor the Group III reagents may be triethyl-M or trimethyl-M, where M isthe Group III element. A suitable metal organic liquid source of Group Vatoms may take the form of tertiary-butyl-N or diethyl-N-hydride, whereN is the Group V element. The high purity carrier gas may be eitherhydrogen, nitrogen or argon. The dopants may also be delivered into theMOMBE growth chamber using metal organic sources. These may bediethyl-beryllium or diethyl-zinc for the p-dopants, beryllium and zinc,respectively, or diethyl-tellurium or tetramethyl-silane for then-dopants tellurium or silicon respectively.

The outlet of the bubble 15 is coupled via the manual needle valve 17 toa branch 33 in the tubing leading respectively to a first air operatedvalve 18 in the path to the reactor line 12 and to a second air operatedvalve 19 in the path to the vent line 13.

As earlier noted, the valves 18 and 19 are ganged to switch to oppositestates, so that when the apparatus is in normal operation, and noreagent is being supplied to the injector, the tubing is conducting thecarrier borne reagent to the vent line. Thus the tubing between thebubbler 15 and the branch 33 and between the branch 33 and the inlet tothe valve 19 contains the carrier borne reagent flowing through. Inaddition, the tubing from the branch 33 to the inlet of the valve 18normally fills with reagent although the contents are not in the path ofprincipal flow during venting.

When it is desired to supply the reagent to the reactor line 12, whichleads to the injector, the valve 19 is shut-off and the valve 18 isopened. Thus, because the tubing from the bubbler to the entrance of thevalve 18 is already substantially filled, the reagent enters the reactorline 12 after an irreduceable delay, set primarily by the volume of thereactor line leading to the MOMBE growth chamber injectors. A transient,which is produced, is minimized by a compensation flow, which willsubsequently be discussed. Care in these particulars affects theaccuracy of metering the supply of reagent to the MOMBE growth chamber.

The pressure and temperature conditions in the manifold are controlledto sustain accuracy in the metering of the MO reagent as it leaves thebubbler and proceeds to the injector while using minimum carrier gasflow. The bubbler output is held at an accurately controlled variabletemperature, typically near 0° C. The temperature is adjusted to controlthe saturated vapor pressure of the reagent, and thereby controls thepressure above the surface of the liquid in the bubbler. Typically thispressure will be within the range of one to 100 torr, as measured by themanometer 16 at the outlet of the bubbler. The needle valve 17, is usedto manually adjust pressure within the bubbler to a desired value abovethe saturated vapor pressure and produces a second reduction in pressureto approximately 0.1 torr in the tubing at branch 33. At the injectorfor the MOMBE growth chamber (shown in FIG. 4), a further reduction inpressure occurs to approximately 10⁻⁵ torr.

In addition to the foregoing controls, the temperature of the manifoldtubing from the outlet of the bubbler 15 to the injector is maintainedat a sufficiently high temperature to prevent condensation of thereagent on the walls of the tubing, which would add uncertainty to themetering of the MO reagent supplied to the growth chamber. Thistemperature is typically from 50° to 80° C. The temperature of thesubstrate within the MOMBE chamber is from 200° C. to 800° C., dependingupon the reaction taking place.

The flow of the MO reagent (Fa in sccm may be mathematically expressedas a function of the carrier gas flow (Fc) in sccm, the saturated vaporpressure (Pa) of the MO reagent in torr and the pressure in the bubblerchamber over the liquid (Pc) in torr. The expression is as follows:

Fa=Fc((Pc/Pa)-1) (1)

One may now consider manifold operation and the practical implicationsof expression 1. Assuming a manifold with operating vacuum pumps and nocarrier gas being supplied, the space over the MO reagent within thebubbler chamber 20 is being evacuated continuously through the needlevalve 17 and through one of the air operated valves 18 or 19 leadingeither to a vent or the growth chamber. The atmosphere over the MOreagent becomes pure MO vapor and the pressure (Pc) in the space overthe MO reagent in the chamber 20 is attributable to the saturated vaporpressure (SVP) of the reagent. Assuming an adequate supply of liquid MO,the flow rate of MO reagent will reach a dynamic equilibrium in which anincrease or decrease in temperature, will effect a proportional changein the vapor pressure of the MO and proportional change in the flow rateof MO reagent to the growth chamber. (The dynamic equilibrium approachesan equilibrium dictated by thermodynamic considerations as the flow rateof the MO reagent approaches zero.)

When a carrier gas, such as hydrogen, is introduced into the bubbler,passing through the MO reagent and sharing the space over the MO liquidwith MO vapor, the atmosphere over the MO liquid becomes a mixture ofcarrier gas and MO vapor. Assuming a low, but fixed rate of introductionof carrier gas, a dynamic equilibrium is reached in the proportion of MOvapor to carrier gas and in the proportion of hydrogen gas dissolved inliquid MO.

The amount of carrier gas dissolved in the liquid MO is normally oflesser importance since the amount going into the solution is small,quickly stablizes, and does not substantially affect the rate of flow ofcarrier gas or MO to the growth chamber.

The proportion of MO vapor to carrier gas in the gaseous mixture isdependent on the evaporation and diffusion mechanisms both dependent inturn on temperature, pressure, and equipment design. Theseconsiderations are of importance since they determine the flow rate ofMO reagent to the growth chamber. For modest rates of flow of carriergas, the carrier gas is efficiently saturated with reagent vapor,however as the flow of carrier gas increases, the carrier gas willeventually leave the bubbler in an unsaturated condition.

The goal of the manifold design, where significant quantities of carriergas are continuously introduced and both carrier gas and MO are removed,is to reach a predictable and stable, steady state condition, in a shorttime, commensurate with the times required to form the various steps inepitaxial processing. This stabilization of the reagent flow is carriedout with the flow switched to the vent line of the manifold.

Evaporation of MO liquid and mixture of the gases is aided by optimizingthe bubbler to provide maximum opportunity to the MO molecules to escapefrom the liquid state through the bubble membranes into the gaseousstate in the bubbles as they percolate up through the liquid reagent.

After passage through the liquid reagent, the bubbler design shouldprovide maximum opportunity for the mixed carrier gas and gaseous MO tocontinue the molecular exchange with the MO liquid through theliquid-gaseous interface at the surface of the liquid. A relativelylarge liquid surface facilitates faster steady state stabilization ofthe proportions of the gas mixture for a given carrier gas flow, andshould insure that steady state conditions are quickly attained at thehighest required rates of carrier gas flows (consistent with the growthchamber pumping capacity, a primary requirement for MOMBE operation).

The arrangement illustrated in FIG. 1 achieves full control of the MOmolar flow by precise control of the hydrogen carrier gas flow (Fc), MObubbler temperature which affects (Pa) and the setting of the needlevalve 17 which controls the bubbler outlet pressure (Pc) (the threevariables in expression 1).

The apparatus, as indicated by expression 1, provides sensitive controlof the MO molar flow rates. Since the vent and reactor lines are held ata low pressure in relation to the partial pressure of the MO reagent,and the partial pressure of the MO reagent is lower than the pressure ofthe carrier gas supply line, the flow of carrier gas is continuous andat a rate set by the mass flow controller. In addition, obstruction ofthe flow of carrier gas by the needle valve N, permits one to controlthe pressure at the bubbler outlet (Pc). The amount of MO reagentcarried by carrier gas may be regulated by adjusting the temperature ofthe liquid MO reagent to vary the vapor pressure (Pa) of the MO reagentin the bubbler. This sets the ratio of MO molecules to carrier gasmolecules in the gas mixture delivered to the reactor or vent lines, thecarrier gas flow being earlier established by the mass flow controller.

The hydrogen carrier gas flow is established at a maximum valueconsistent with maintaining the desired vacuum (10⁻⁴ torr) in the growthchamber. While the number may vary somewhat from pumping system topumping system, this sets a practical upper limit upon carrier gas flowof approximately 100 sccm. Customary carrier gas flows are from about 10to 40 sccm for reasonable process speeds.

With low vapor pressure materials, such as TEAl, the settings of themass flow controller for carrier gas flows (Fc) will ordinarily berequired near the maximum permissible. Much lower carrier gas flows willbe appropriate with high vapor pressure materials.

The ability to both raise the temperature to increase partial pressure(Pa) and to lower the pressure (Pc) at the bubbler outlet to a valueabove, but near to the vapor pressure gives one maximum freedom toaccommodate low vapor pressure materials.

For example, with the triethyl-aluminum (TEAl) reagent, an elevatedtemperature of 50° C. produces a partial vapor pressure (Pa) of only0.35 torr. The Pc under these circumstances, which is varied byadjusting the needle valve, may be set to produce pressures varying from6 to 1 torr, to produce MO reagent flows of from 10¹⁷ to 4×10¹⁹molecules per minute.

Ordinarily molar flow rates of from 10¹⁶ to 10¹⁹ molecules per minuteprovide desirable growth rates over customary wafer dimensions.Desirable growth rates are those which permit the process to becompleted with appropriate speed, and which permit accurate control ofepitaxial thicknesses.

The manifold is adjusted to use the appropriate bubbler temperatureconsistent with these objectives, avoiding dissociation of the MOcompounds within the bubbler. A range of from 50° C. to 80° C.represents the upper limit to the bubbler temperature with low vaporpressure reagents. Lower temperatures in the range of from 0° C. to 30°C. are preferred, and may be used with many common reagents.

The pressure in the bubbler is set above the vapor pressure of the MOreagent, normally at least double the vapor pressure, and substantiallyless than atmospheric pressure in the interest of maximizing therichness of the mixture of MO reagent to carrier gas.

Reducing the bubbler pressure Pc, with adequate heating to increase thevapor pressure (Pa) of the MO reagent therefore permits delivery ofoptimum molecular flow rates while using minimum carrier gas flow.(These measures are particularly useful for low vapor pressure materialswhich would otherwise require very large carrier flows, likely to beruled out because of the pumping speed limitations of MOMBE equipment.)

The bubbler pressure PC is a powerful parameter in controlling the molarflow of the MO reagent. For instance, a variation from 6 to 1 torrproduces a variation from 10¹⁷ to 4×10¹⁸ molecules of triethylaluminum(TEAl) per sccm of carrier with a bubbler temperature of 50° C. and a Paof 0.35 torr. Similarly, a variation from 230 to 20 torr produces avariation of from 10¹⁷ to 4×10¹⁹ molecules of triethylgallium (TEGa) persccm of carrier, with a bubbler temperature of 40° C. and a Pa of 14torr.

The achieveable accuracy of the molar flow rates is ±0.5% in the presentthree parameter control apparatus. For a given bubbler temperature, thecarrier flow is first set and then the needle valve "N" is adjusted togive the desired value of pressure in the bubbler. The mass flow (Fc) iscontrolled to ±0.5%. The temperature is controlled to ±0.01° C.providing a control of the vapor pressure of the MO reagent of 0.05%.

The use of a three parameter control of MO flow therefore gives thepresent MOMBE manifold substantial flexibility. The arrangement allowsthe controlled delivery of useful molar flows of MO reagents having awide range of vapor pressures without resorting to high sourcetemperatures or high carrier gas flows.

The simplified arrangement illustrated in FIG. 1 is designed to maintainsubstantially equal flows of carrier gas through the bubbler with thecarrier gas being supplied to the vent line or the reactor line. Inpractice, the vent lines and the reactor line experience increases anddecreases in pressure as this switching takes place which affects Pc andtherefore the accuracy of metering the molar flows of MO reagent. Thearrangement illustrated in FIG. 2 provides a method of switchedcompensation flows which maintains the pressures in the vent and reactorlines substantially constant and equal as gas switching takes place andremoves instability due to changes in pressure in the reactor and ventlines. It also removes instability due to changes in pressure in thecarrier gas supply line.

The FIG. 2 arrangement comprises a "source flow" to which a parallelledcompensation flow has been added. The source path leads from the carriergas line 11 via the bubbler to the reactor and vent lines 12 and 13respectively, and is similar to that illustrated in FIG. 1 except forthe provision of additional valves 26, 27 and 28 about the bubbler,which are useful in setting up the manifold for operation. Thecompensation path comprises a mass flow controller 25 having its inletconnected to the carrier gas line and its outlet connected via a Tee 34to the inlets of two valves 23 and 24. The outlets of the valves 23 and24 are connected respectively to the reactor line and the vent line.

In addition, the compensation path valves 23 and 24 in the FIG. 2arrangement are ganged with the "source" valves 18 and 19 in the bubblerpath in an opposite sense. In particular, the valve 23 is arranged toopen when the valve 19 opens and to close when valve 19 closes.Similarly, the valve 24 is designed to open when the valve 18 pens andto close when the valve 18 closes. Consistent with opposite states insource and compensation paths, the closing of valves 19, 23 isaccompanied by the opening of valves 18, 24, and vice versa. Thereversal of connections of the source and compensation paths to thereactor and vent lines, and the ganging of the four valves causes theflow in the source path (via the bubbler) to be to the vent line, whenthe flow in the compensation path is to the reactor line (and viceversa).

The four valves are thus operated simultaneously so that when the sourceflow via the bubbler is added to the reactor line and the equivalentcompensation flow is subtracted from the reactor line and the flows areadjusted to be equal, the total flow, and the pressure in both thereactor and vent lines remains constant during switching effectivelyeliminating the flow transients. In addition, the flow in the carriergas line is also stabilized. Experience with this approach indicatesgreater stability than with an automatic pressure control system (APC).The latter, which provides pressure control after a small time delay ina feedback loop during which the transients are stabilizing tends tohave a greater switching discontinuity than occurs when the pairedpneumatic valves are switched to opposite states.

A more extensive view of a MOMBE manifold is illustrated in FIG. 3. Heretwo bubblers 15 and 35 are provided for supplying EEGa and TEAl,respectively, to an injector to the MOMBE growth chamber. It should beunderstood that other MOMBE manifolds may be require to supply the otherinjectors in a growth chamber. Typically there will be at least fourinjectors, each usually requiring a manifold. One injector and manifoldwill be provided for Group III MO reagents, and one injector andmanifold for Group V MO reagents. An injector, not necessarily entailinga bubbler manifold may be provided for p-dopants and another injectornot necessarily entailing a bubbler manifold may be provided forn-dopants.

Returning now to FIG. 3, the illustrated MOMBE manifold is formed of twosubstantially identical portions, each portion formed about one of thebubblers 15 and 35. A single MOMBE manifold may conveniently accommodatetwo or three "bubblers". Ordinarily, if more than three otherwisecompatible MO reagents are desired, a second manifold will be providedcoupled to another injector. A typical growth chamber provides at leasteight injectors, each of which may be fed from manifolds of the typeherein disclosed.

What is claimed is:
 1. In a Metal Organic Molecular Beam Epitaxy (MOMBE)apparatus, the combination comprising:A. a manifold including(1) asupply line for carrier gas under pressure, (2) a first and a secondadjustable mass flow controller with an input and output, the inputsbeing coupled to said supply line for admitting said carrier gas atdesired rates of flow (Fc), (3) a bubbler with a chamber for avaporizable liquid reagent having an inlet coupled to said secondcontroller output, opening beneath the chamber filling level forbubbling carrier gas through liquid reagent and an outlet disposed abovesaid level to remove carrier gas borne reagent, and an adjustabletemperature control for establishing the vapor pressure (Pa) of saidliquid reagent, (4) pressure sensing means coupled to the bubbler outletto sense the pressure (Pc), (5) an adjustable needle valve having a highpressure input port, a low pressure output port, the high pressure inputport being coupled to said bubbler outlet, the needle valve adjustmentsetting the outlet pressure at a desired value, the foregoing elementsproviding three parameter control of carrier borne reagent flow (Fa) tothe low pressure port of said needle valve in accordance with thefollowing expression

    Fa=Fc/(Pc/Pa-1)

(6) a vent line, (7) a reactor line, and (8) four ganged valves (24, 18;23, 19), the first pair of valves (24, 18) opening together as thesecond pair of valves (23, 19) closes together, and the first pairclosing together as the second pair opens together, the first valves(24, 23) in the first and second pairs each having one port coupled tothe output port of said first mass flow controller and the other port tosaid vent line and reactor line, respectively, the second valves (18,19) in the first and second pairs each having one port coupled to theoutput port of said needle valve and the other port to said reactor lineand vent line, respectively, adjustment of said mass flow controllersfor equal gas flows maintaining the total flow of gas into said vent andreactor lines substantially constant to minimize valve switchingtransients, and B. a MOMBE growth chamber having(1) an injector to whichsaid reactor line is connected, (2) means to support a substrate uponwhich an epitaxial layer is to be formed within said chamber, (3) meansto heat said substrate to facilitate formation of said epitaxial layerwith said carrier gas borne reagent, and (4) means to evacuate saidgrowth chamber at a rate which will maintain said growth chamber at alow pressure during admission of carrier gas at which substantially allof the molecules of carrier gas borne reagent, which impact thesubstrate, do so without prior collision.
 2. The arrangement set forthin claim 1 whereinthe pressure in the carrier gas input line is lessthan atmospheric pressure, the pressure at the bubbler outlet is set toexceed the vapor pressure of contained MO reagent, and to be less thanthe pressure at the carrier gas inlet line, and the pressure in thegrowth chamber is 10⁻⁴ torr or less to facilitate the continuous flow ofcarrier gas from supply line to vent or reactor line.
 3. The arrangementset forth in claim 1 whereinthe bubbler temperature is upwardlyadjustable short of dissociation of the MO reagent to increase thepartial pressure (Pa) of a liquid MO reagent, and the bubbler outletpressure (Pc) is downwardly adjustable toward the vapor pressure of theliquid MO reagent to increase the richness of the mixture of MO vapor tocarrier gas, to allow the manifold to process MO reagents having lowvapor pressures at reasonable flow rates.
 4. In an epitaxial processingapparatus, the combination comprising:A. a manifold including(1) asupply line for carrier gas under pressure, (2) a first and a secondadjustable mass flow controller with an input and output, the inputsbeing coupled to said supply line for admitting said carrier gas atdesired rates of flow (Fc), (3) a bubbler with a chamber for avaporizable liquid reagent having an inlet coupled to said secondcontroller output opening beneath the chamber filling level for bubblingcarrier gas through liquid reagent and an outlet disposed above saidlevel to remove carrier gas borne reagent, and an adjustable temperaturecontrol for establishing the vapor pressure (Pa) of said liquid reagent,(4) pressure sensing means coupled to the bubbler outlet to sense thepressure (Pc), (5) an adjustable needle valve having a high pressureinput port, a low pressure output port, the high pressure input portbeing coupled to said bubbler outlet, the needle valve adjustmentsetting the outlet pressure at a desired value, the foregoing elementsproviding three parameter control of carrier borne reagent flow (Fa) tothe low pressure port of said needle valve in accordance with thefollowing expression

    Fa=Fc/(Pc/Pa-1)

(6) a vent line, (7) a reactor line, and (8) four ganged valves (24, 18;23, 19), the first pair of valves (24, 18) opening together as thesecond pair of valves (23, 19) closes together, and the first pairclosing together as the second pair opens together, the first valves(24, 23) in the first and second pairs each having one port coupled tothe output port of said first mass flow controller and the other port tosaid vent line and reactor line, respectively, the second valves (18,19) in the first and second pairs each having one port coupled to theoutput port of said needle valve and the other port to said reactor lineand vent line, respectively, adjustment of said mass flow controllersfor equal gas flows maintaining the total flow of gas into said vent andreactor lines substantially constant to minimize valve switchingtransients.
 5. The arrangement set forth in claim 4 whereinthe pressurein the carrier gas input line is less than atmospheric pressure, thepressure at the bubbler outlet is set to exceed the vapor pressure ofcontained M reagent, and to be less than the pressure at the carrier gasinlet line, and the pressure in the growth chamber is 10⁻⁴ torr or lessto facilitate the continuous flow of carrier gas from supply line tovent or reactor line.
 6. The arrangement set forth in claim 4 whereinthebubbler temperature is upwardly adjustable short of dissociation of theMO reagent to increase the partial pressure (Pa) of a liquid MO reagent,and the bubbler outlet pressure (Pc) is downwardly adjustable toward thevapor pressure of the liquid MO reagent to increase the richness of themixture of MO vapor to carrier gas, to allow the manifold to process MOreagents having low vapor pressures at reasonable flow rates.